Are people with six packs actually strong?
Like so many other things...it depends
We all know that everyone technically has a six pack, it just doesn't show for most people because their body fat is covering it up. Having a six pack is simply a matter of low body fat and not, as one of these other answers is contending, a matter of doing enough sit ups and deadlifts. Doing lots of heavy deadlifts CAN make your six pack show up maybe 1% higher bodyfat than usual, but it's not going to make anywhere close to the difference that a proper diet will.
This is Justin Bieber. Does he look strong to you?